A motor is used to rotate media in conventional mass storage devices such as compact disc (CD) or digital versatile disc (DVD) drives, as well as magnetic hard disc drives and floppy disc drives. To obtain an acceptable degree of precision for assuring that data can be correctly written to or read from the media, it is desirable to obtain accurate information about the rotational speed of the motor during operation.
Methods for sensing the speed of a motor include the use of a codewheel coupled to the motor. Codewheels include discs typically constructed from metal, film, or glass, the discs having a pattern of slits or bars. A codewheel may be mounted on the shaft of the motor, perpendicular to the axis of rotation, such that the codewheel is rotated by the motor shaft. Codewheels may also be remotely coupled to the motor by one or more gears. For reading the codewheel, an encoder module can be mounted in a fixed position, relative to the rotating codewheel. The encoder module includes a sensor to optically sense the motion of markings on the codewheel as the motor shaft rotates. The module produces a digital output which may be used to determine the speed of the motor.
In another solution, used in some CD and DVD drives, a pattern is marked directly upon the media (e.g., by silk screening or embossing a pattern or codewheel onto each CD or DVD). As the motor rotates the CD or DVD, the encoder module senses the motion of the marks on the media, and produces a digital output which may be used to determine the speed of the motor.